Can anyone advise me on how to maintain a very small leaf size on this species. I have fed the tree only on fish emulsion, keeping off the Nitrogen to help stop the leaves getting too big and to avoid thickening in the wrong places.

The biggest leaves at present are about one inch long with some of the others down to about half an inch.

I assume you mean Cotoneaster symonsii, which is a synonym of C. horizontalis. There is a lot of information on C. horizontalis as bonsai on the Internet. Bonsai4Me has a progression series & there are numerous others. Being the owner of one Cotoneaster (so limited experience), I would imagine that the standard procedures would be appropriate:As much sun as it will tolerate.Frequent pinching and pruning.Cotoneaster does not appear to be a candidate for defoliation.I would not use fish emulsion as the only fertilizer. Plants like it, but it is not a reliable source of trace elements. I'm very puzzled at your statement about nitrogen. Your fish emulsion must be different from ours. Fish emulsion in this country is usually relatively high in nitrogen. Does the package list the exact percentage of N-P-K? It is required in this country. Plants need some nitrogen, even when you are trying to keep the leaves small. You might do well to alternate the fish emulsion with something labeled "bloom booster." You could also use 0-10-10 during periods of active growth. Make sure that one of the fertilizers you use, say once a month, contains trace elements (micro-nutrients).

My experience with Cotoneaster is also limited, but I just try to grow out a trunk and overall tree size to match the leaves rather than try to "dwarf" the leaves through bonsai technique..

Example is a Coral Beauty that I picked up about a month ago, a nice inch thick trunk on it but the leaves are say 3/4 inch long, a bit on the larger size compared to other cultivars like Streibs findling-(C. dammeri). So what I did was just stick it in the ground to grow it out to maybe double what it is now and we'll talk again in say two more years. By then hopefully my trunk diameter will be a bit bigger and will help with maintaining a better proportion with those leaves. For now I enjoy the sweet anticipation of one day working with the plant.

I use fish and seaweed emulsion blended together and all the trees seem to really like it Neptunes Harvest blue label.-Jay